Virtual IP's from multiple subnets on a single Interface... what happens?

[Adam Garsha]

HP-UX network admin documentation states that you can have virtual IP's from multiple subnets attached to a single interface.

Can you think of a reason why you would ever be able to make use of multiple subnetted virtual IP's attached to a single interface? If not, then should the tools stop you?

If you did alias a virtual IP from a different subnet to an interface that already contained multiple active/functioning virtual IP's from a single subnet what would happen?

Would all packets that go out the interface be hopelessly lost? i.e. no outgoing traffic? what would outgoing connections use for the source IP? the lowest lanX:0 IP or would they take on the higher lanX:N (Where N>0) IP for the source?

Very curious and sorry for asking dumb questions, I am just very curious.

[Adam Garsha]

P.S. I am a point givin' fool...

[Bill Costigan · Accepted solution]

I use this quite often in our lab. It really isn't a problem.

Basically the system looks kind of like a router. You have multiple IP addresses sharing the same mac address.

A router sends and receives packets for multiple networks through the same interface. It simply looks at the IP header of the packet and routes it to the correct stack.

Assume we had a HPUX configured as:
lan0 192.168.1.50
lan0:1 172.16.10.60

any packets sent to a 172.16.10.0 address would leave the I/F with the I/F's mac and a source IP of 172.16.10.60. Packets to a 192.168.1.0 address would use 192.168.1.50 as its source address.

If the system needed to go to a router with an address of 192.168.1.200, it would use the 192.168.1.50 address.

The system would respond to ARPs for either IP with the same mac address. All incoming packets would have the destination IP checked and the packet would be routed to the correct internal IP stack

[Mel Burslan]

well, yes you are right, the interface itself acts like a router but how does the packets get treated where they arrive at, i.e., on a switch port ? Switches, even though they are called hubs with a router embedded, are not that clever to respond to different IP stacks. Or am I wrong fundamentally ?

[rick jones]

There is nothing that says one cannot run multiple IP subnets in the same broadcast domain. The switches, being layer-2 devices, will be blythly unaware of anything happening at layer 3 (network, ie IP).

There is no reason the commands in HP-UX (what I presume you mean by "the tools") should preclude it.